1. Field of the Invention
The present invention relates to a method and apparatus for pre-determining the withdrawal/release force required to disconnect an appliance coupler such as a plug and socket. The invention also relates to a method and apparatus for preventing disconnection of an appliance coupler such as a plug from a socket. Furthermore, the invention relates to method and apparatus for preventing inadvertent or unintentional disconnection of a plug from a socket for the supply of electrical power to a medical apparatus.
2. Discussion of Related Art
An appliance coupler enables the connection and disconnection at will, of a cord to an appliance or other equipment and it consists of two parts: a connector and an appliance inlet. Often the connector takes the form of a plug while the appliance inlet takes the form of a socket. Typically the cord is intended to deliver electrical power such as AC or DC current to the appliance. Alternatively, a cord may be intended to serve as the conductor for the transmission of data.
It is common that where an electrical connection is effected by utilization of a plug and socket, the combination must be capable of being connected and disconnected by the plug being inserted and withdrawn from the socket with the use of no more than a strength or force which may be easily exerted by unaided manual effort. The minimum force required to disconnect the plug and socket may be referred to as the withdrawal force. The withdrawal force is exerted by way of a pull force which is a force applied to the plug and socket combination which tends to separate the connection. Notwithstanding the ease by which a plug may be disconnected from a socket, there may be situations such as when the apparatus is operating where the plug is required to withstand a pull force that is significantly greater than the withdrawal force. These otherwise conflicting requirements may be satisfied by the provision of a retaining component that operates independently of the retaining effect achieved by the otherwise unaided plug and socket combination.
Generally, the plug and corresponding socket are configured to slidably engage one another, the socket having slots to a depth of at least the length of the pins. The pins may protrude from a support structure or be integral with the support structure. The pins may be constructed from a conductor, such as metal or some combination of support structure having a conductive component. The slots are generally housed within a structure having insulating properties. Generally there is a frictional retaining force between the pins and their corresponding slots. In addition, in some plug and socket arrangements, the housing of each of the respective plug and socket provides a frictional retaining force. This frictional retaining force will tend to oppose a pull force and thus contribute to the level of withdrawal force required to cause a disconnection to the plug and socket.
In one form commonly used in Australia for transmission of electrical power between the wall socket and the cord, the relevant standard mandates the use of a set of three pins in the plug. One pin may be used as an “earth” or ground, as known in the U.S., the other two pins may be respectively “active” and “neutral”. The pins are generally flat and rectangular, having approximate dimensions of 1.5 mm thick×20 mm long×5 mm wide. The pins are arranged around a central point. An earth pin is arranged approximately radially to the central point, whereas the other two pins are arranged generally tangentially. The slots within the socket are arranged to receive the pins of the plug, there being a corresponding slot for each pin. In this way, there is a unique orientation for engaging the plug and socket together. In another form commonly used in Australia for transmission of electrical power, only two pins are used, the earth pin being omitted. In some European countries, a set of two cylindrical pins is used. In the United States, plugs having a generally cylindrical earth (ground) pin and two generally flat rectangular pins are commonly used. Other arrangements of pins and sockets are known.
There are international standards for the point of connection between the cord and the electrical appliance. Those standards often include a specified withdrawal force for the plug and socket combination.
In another form of electrical power cord, the cord has a wall socket engaging plug at one end and a device engaging housing at the other end. FIG. 1 shows a known device engaging housing 10. The device engaging housing 10 is generally rectangular and has a first end connected to the cord and a second end 12 for connection with an apparatus. The second end 12 has a generally irregular hexagonal “extruded” profile. The length of the “extrusion” is approximately 19 mm and the length of the device engaging housing 10 is approximately 55 mm. In use, the device engaging housing 10 is positioned in front of a socket 20 having a negative generally irregular hexagonal shape, the socket 20 being positioned on the exterior of the apparatus 5 and sided therein. FIG. 2 shows a known socket. The device engaging housing 10, can generally be inserted for the length of the generally irregular hexagonal profile, which is to say, approximately 19 mm. There are three flat rectangular pins 25 within the socket 20 of the apparatus which slide into corresponding slots 15 within the device engaging housing 10. A retaining force between the device engaging housing 10 of the cord and the socket 20 of the apparatus is provided by a frictional force between engaging surfaces, such as (i) the exterior walls 17 of the second end 12 of the device engaging housing 10 and the interior walls 27 of the socket 20 within the apparatus; and (ii) the exterior surface of the pins 25 and the interior walls of the slots 15. In the case of the plug and socket depicted in FIGS. 1 and 2, the withdrawal force is relatively low, since the only retaining force is due to friction between the plug and socket.
Other fields also use arrangements of plugs and sockets. In the field of data communication, for example via a telephone network, it is known to provide plug and corresponding socket sets which include a retaining device. Such an arrangement is depicted in FIGS. 3(a)–3(c) and FIGS. 4(a) and 4(b). FIGS. 3a, 3b and 3c depict front, side and top view respectively of a known plug 30. FIGS. 4a and 4b depict front and side views respectively of a known socket 40. Such a plug and socket combination is general known as RJ series connectors.
Plug 30 includes a cantilevered arm 32 and is able to pivot about a pivoting point when subject to a force. The arm 32 is resiliently biased in an upper position. As best shown in FIG. 3(c), the arm 32 has a wide portion 34 and a narrow portion 36. At the junction between the wide and narrow portions 34, 36 is a pair of shoulder regions 38.
Socket 40 is adapted to slidingly receive within it plug 30 and hence socket 40 has a shape generally complementary to the plug 30. Socket 40 includes a shoulder 48 adapted to engage the shoulder 38 of the plug 30. Whilst plug 30 is being inserted into socket 40, the cantilevered arm 32 must pivot into a lower position. Once the plug 30 is fully inserted into socket 40, the arm 32 springs back into the upper position and the respective shoulder regions 38 and 48 engage one another. Hence the plug 30 is retained within the socket 40. The plug 30 and socket 40 will not disengage until the arm 32 is depressed into the lower position, disengaging the two shoulders 38 and 48. In some situations where the extreme pull force is applied to the cable or plug 30, the arm 32 may break off or be damaged by permanent deformation.
Another known arrangement from the field of data communications for retaining plugs and sockets together is depicted in FIGS. 5 and 6. In this arrangement, pins 54 located within the plug 50 are arranged to slidingly engage with slots 64 within the socket 60. In this arrangement, plug 50 and socket 60 are primarily retained via screws 52 in the plug 50 which are adapted to engage with corresponding slots 62 having a thread complementary to the screws 52. In addition, a friction fit occurs between the pins and slots and between the sheath 55 and the corresponding inner surfaces of socket 60. Disconnecting this plug and socket combination without first unscrewing each screw 52 would result in damage to the screws or their reciprocal threaded bores or the integrity of the plug and socket or the fixture of the socket to the attached apparatus.
A problem with the known arrangements for retaining the connectors and appliance inlet together is that the withdrawal force is either too low to satisfy some operation situations in that the connector disconnects from the appliance inlet when subject to pull forces that are often encountered in the operating environment. Alternatively, the withdrawal force is so high that physical damage may result to the connector and appliance inlet before the connector disconnects from the appliance inlet. For example, in the case where a screw is used to hold the connector and appliance inlet together, other parts of the connector and appliance inlet may break before the screw disengages. Where such an arrangement to be used for power cables, it may be that live wires break or become exposed to the environment before the screw disengages or the appliance may be otherwise damaged. As a further undesirable consequence, the connector may separate from the attached power cord or the appliance inlet may separate from the rest of the appliance. In each instance the separation of components may cause short circuits or even live electrical leads to be exposed to the environment thereby giving rise to a situation where further appliance damage, electrocution, arcing and ignition of fire may occur. Further, the power cord, connector, appliance inlet or retaining device may become damaged and rendered in a condition that would be unsuitable for further use.
In addition to the general standards for appliance couplers, additional standards proclaimed by international or national standards organizations or by sectional bodies (such as those responsible for setting medical apparatus standards) may require a unique withdrawal force to be implemented in particular applications. For example, see proposed standard ISO/TC 121/SC 3N 1066 titled Lung Ventilators and Related Equipment dated Jul. 8, 2001 published by International Organization for Standardization (ISO).
In other instances, it may be desirable that where a set of cords are arranged in series and connected by complementary plugs and sockets engaging with each other, that the engaged complementary plugs and sockets are able to withstand a pull force up to a specified limit without disconnecting. While such a connection may be achieved, by not ensuring that there is a maximum force above which the connected pins and sockets will disengage there is the risk that a sufficiently high pull force will cause damage to the components. The present invention may be used in the connection of sets of cords.
A problem with the known plugs and sockets described above, and by way of example only as depicted in FIG. 1 and FIG. 2 is that the retaining force between the plug and socket due to friction between corresponding complementary slidingly engaged surfaces does not meet the regulatory requirement of withstanding an industry or apparatus specific standard standards. In this example, the standard force would be set at a minimum, for example, to prevent inadvertent disconnection. In some industries, this may have a set standard of, for example, 100 Newtons. This is because the plug and socket combination are intended to stay connected during normal use while allowing for their disconnection to occur by the application of reasonable manually applied force. That is to say the plug and socket combination should be capable of being connected and disconnected without the need for the exertion of force that is greater than might reasonably be applied by a user without assistance. Moreover, the plug should not be designed such that the withdrawal force exceeds the strength limits of the plug and associated wires. Thus, for example, the maximum withdrawal force may be set, for example, at 300 Newtons.
A preferred aim of the present invention is to satisfy a requirement that a detachable cord with a plug withstand a pull force (defined as an axial pull of force) of the magnitude of, for example, greater than 100 to less than 300 Newtons, but still be easily disconnected from a medical appliance by a user.
Another problem with the known plugs and sockets described above, and as depicted in FIG. 1 and FIG. 2 is that the retaining force between the plug and socket due to friction between corresponding complementary slidably engageable surfaces may be unpredictable in a mass produced componentry.
The multiplicity of safety and regulatory standards that may apply to a plug and socket combinations make it particularly difficult for achieving a single plug and socket combination which will meet different withdrawal force standards in different appliance applications or in different countries. Furthermore, while a standard may allow for the permanent attachment of a power cord or other electrical conductor cord to the appliance, from a manufacturer's perspective it is desirable to allow for the interchangeability of power cords through adoption of an appliance coupler so as to facilitate production and distribution of systems to satisfy a number of standards.